A.L wrote:
How many times in a week did he make its athlete do the 50/50 sprints?
0 times.
Apologies for the slight snark- but Lydiard didn't MAKE his athletes do anything. It was kind of a big part of his overarching philosophy. He might have been brusque, but he wasn't the coach barking orders and forcing things. He was the guy who'd watch you get three reps in and say "shut it down, you're not right today, try again tomorrow."
From what I understand, the 50/50s were just sharpeners- you'd do it maybe once or twice before you started racing, and once you had started racing, the races themselves were the race-specific training. I see much less of a need for those kind of workouts in a typical American high school schedule, where kids are racing 2-3 times a week anyway. You could probably get a kid just as sharp by having him race under-distance races in dual meets. Throw the two-miler in the 800, let the milers and half-milers race the 400 or the 4x400, etc.
With regard to the speed of the floats, they should be pretty quick- Walker and his ilk would run two miles in this fashion around 9:00 +/-. Reportedly, Walker once did 8:48 (or, about 30 sec slower than a flat out deuce). I've done stuff like this before to prepare for racing and it is BRUTAL.
Longer distance guys might even do 5k of this type of training- in a Sports Illustrated profile, Kenny Moore reports Viren doing 5000m of sprint/floats within ~20 seconds of his 5k PR! That's the kind of vicious work that a proper Lydiard base allows.
As most of us don't do the 'proper base' work, we should probably temper our sharpening efforts...